A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implanted stimulation processor 108. Besides receiving the processed audio information, the stimulation processor 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
In cochlear implants today, a relatively small number of electrodes are each associated with relatively broad frequency bands, with each electrode addressing a group of neurons through a stimulation pulse the charge of which is derived from the instantaneous amplitude of the envelope within that frequency band. In some coding strategies, stimulation pulses are applied at constant rate across all electrodes, whereas in other coding strategies, stimulation pulses are applied at an electrode-specific rate.
One problem with cochlear implants is spatial channel interaction where there is considerable geometric overlapping of electrical fields at the location of the excitable nervous tissue when multiple stimulation electrodes are activated the same neurons are activated when different electrodes are stimulated. Spatial channel interaction is primarily due to the conductive fluids and tissues surrounding the stimulation electrode array. Such spatial interaction acts as channel crosstalk which smears audio information across channels and hampers place pitch perception. Up to now the problem of channel interaction has been addressed using coding strategies and/or electrode designs that try to focus the electrical field to the site of stimulation. Such approaches consume a relatively high amount of electrical power.
One prior art approach is known as phased array stimulation, which produces focused stimulation in a medical stimulation device, as described, for example, in U.S. Patent Publication 2006247735, which is incorporated herein by reference. Focused intracochlear electric stimulation is produced using an array of N electrodes: “For each electrode site, N weights are computed that define the ratios of positive and negative electrode currents required to produce cancellation of the voltage within scala tympani at all of the N−1 other sites. Multiple sites can be stimulated simultaneously by superposition of their respective current vectors.”
Another prior art approach uses a channel interaction matrix where current spread is reduced by simultaneous pulses on multiple electrodes, as described, for example, in U.S. Pat. No. 7,110,821, which is incorporated herein by reference. Positive and negative amplitudes can occur on different simultaneous stimulated electrodes, resulting in short-circuit currents. The stimulation pattern is calculated by a channel interaction matrix that is derived from telemetry measurements: “The measured or estimated channel interaction is collected or compiled and saved as a channel interaction matrix. The channel interaction matrix is created during a fitting procedure by stimulating one channel at a time while measuring the effects of the stimulation on the neighboring channels. The superposition principal is used, as needed, to determine all the terms of the channel interaction matrix.”
U.S. Pat. No. 6,304,787 attempts to diminish current spread by the design of electrode contacts: “In accordance with one important aspect of the present invention, the exposed electrode contacts on the surface of the electrode array have a shape, geometry, or makeup that aids in controlling the current flow and current density associated with the electrode contact as a function of position on the electrode contact. For example, in one embodiment of the invention, the shape or geometry of the exposed electrode contacts is designed to diminish the surface of the electrode contact at the outside edges of the contact, thereby focusing most of the current to flow through the center of the electrode contact.”